This is your body's natural rhythm. It's nothing to be worried about. I usually wake up at 1:30 am to pee and then go back to sleep. If nobody posts about this in a clearer sense before I do I'll get you an article describing the phenomenon.
This is described as segmented sleep and is common in pre-industrial societies who go to sleep at good hours like you and sleep in darkness like you.
"Until the modern age, most households had two distinct intervals of slumber, known as "first" and "second" sleep, bridged by an hour or more of quiet wakefulness. Usually, people would retire between 9 and 10 o'clock only to stir past midnight to smoke a pipe, brew a tub of ale or even converse with a neighbor."
New York times article: http://www.nytimes.com/2006/02/19/opinion/19ekirch.html?_r=2
Also interesting, Craked "7 things you didn't know you were doing wrong" which linke me to the NYT article and has another reference about sleep: http://www.cracked.com/article_19121_7-basic-things-you-wont-believe-youre-all-doing-wrong.html